Method and apparatus for reducing charge density on a dielectric coated substrate after exposure to large area electron beam

ABSTRACT

Embodiments in accordance with the present invention relate to a number of techniques, which may be applied alone or in combination, to reduce charge damage of substrates exposed to electron beam radiation. In one embodiment, charge damage is reduced by establishing a robust electrical connection between the exposed substrate and ground. In another embodiment, charge damage is reduced by modifying the sequence of steps for activating and deactivating the electron beam source to reduce the accumulation of charge on the substrate. In still another embodiment, a plasma is struck in the chamber containing the e-beam treated substrate, thereby removing accumulated charge from the substrate. In a further embodiment of the present invention, the voltage of the anode of the e-beam source is reduced in magnitude to account for differences in electron conversion efficiency exhibited by different cathode materials.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a non-provisional application of and claims thebenefit of U.S. Provisional Application No. 60/558,009, filed on Mar.30, 2004, which is herein incorporated by reference in its entirety forall purposes.

BACKGROUND OF THE INVENTION

Semiconductor device geometries have dramatically decreased in sizesince integrated circuits were first introduced several decades ago, andall indications are that this trend will continue on. Today's waferfabrication plants are routinely producing devices having 0.25 μm andeven 0.18 μm feature sizes, and the plants of the future will soon beproducing devices having even smaller geometries.

As device sizes become smaller and integration density increases, oneissue that has become an increasing concern to semiconductormanufacturers is that of inter-level “crosstalk.” Crosstalk is theundesired coupling of an electrical signal on one metal layer ontoanother metal layer, and arises when two or more layers of metal withintervening insulating or dielectric layers are formed on a substrate.Crosstalk can be reduced by moving the metal layers further apart,minimizing the areas of overlapping metal between metal layers, reducingthe dielectric constant of the material between metal layers andcombinations of these and other methods. Undesired coupling ofelectrical signals can also occur between adjacent conductive traces, orlines, within a conductive layer. As device geometries shrink, theconductive lines become closer together and it becomes more important toisolate them from each other.

Another issue that is becoming more of a concern with decreasing featuresizes is the “RC time constant” of a particular trace. Each trace has aresistance, R, that is a product of its cross section and bulkresistivity, among other factors, and a capacitance, C, that is aproduct of the surface area of the trace and the dielectric constant ofthe material or the space surrounding the trace, among other factors. Ifa voltage is applied to one end of the conductive trace, charge does notimmediately build up on the trace because of the RC time constant.Similarly, if a voltage is removed from a trace, the trace does notimmediately drain to zero. Thus high RC time constants can slow down theoperation of a circuit. Unfortunately, shrinking circuit geometriesproduce narrower traces, which results in higher resistivity. Thereforeit is important to reduce the capacitance of the trace, such as byreducing the dielectric constant of the surrounding material betweentraces, to maintain or reduce the RC time constant.

Hence, in order to further reduce the size of devices on integratedcircuits, it has become necessary to use insulators having a lowdielectric constant. And as mentioned above, low dielectric constantfilms are particularly desirable for premetal dielectric (PMD) layersand intermetal dielectric (IMD) layers to reduce the RC time delay ofthe interconnect metallization, to prevent crosstalk between thedifferent levels of metallization, and to reduce device powerconsumption.

The traditional insulator used in the fabrication of semiconductordevices has been undoped silicon oxide. Undoped silicon oxide filmsdeposited using conventional CVD techniques may have a dielectricconstant (k) as low as approximately 4.0 or 4.2. Many approaches havebeen proposed for obtaining insulating layers having a lower dielectricconstant. Amongst these have been fluorine-doped silicon oxide filmsthat may have a dielectric constant as low as 3.4 or 3.6. Anotherapproach has been the development of carbon-doped silicon oxide (CDO)films. In some cases, CDO films are treated with e-beam radiation duringand/or after growth in order to improve the film properties.

The use of electron beam (e-beam) radiation to treat materials is wellknown. For example, e-beams have been used for curing interlayerdielectrics for microelectronic devices, photoresist exposure, alteringsolubility characteristics of thin film layers, and the like. Often, theelectron sources utilized in the past to generate e-beams for suchelectron beam treatments have been electron guns, which produce e-beamsof narrow cross-section. For some applications, it is desirable toprovide a large-area e-beam source which is controllable, uniform,insensitive to poor vacuum, and long lived. Thus, large area e-beamsources have been developed, some of which are suitable for use insemiconductor processing applications. An example of such a large-areae-beam source is described in U.S. Pat. No. 5,003,178, incorporatedherein by reference in its entirety for all purposes.

When such a large area e-beam is used during a semiconductor fabricationprocess, charge buildup can occur in the materials present on thesemiconductor substrate. For example, charge may buildup in dielectriclayers deposited on the semiconductor substrate. Excessive chargebuildup may result in unwanted electrical effects, including electricalbreakdown across fragile structures such as MOS gate oxides, resultingin possible damage to the semiconductor devices.

Therefore, there is a need in the art for methods and structures whichreduce the buildup of charge during electron beam treatment ofsemiconductor substrates.

SUMMARY OF THE INVENTION

Embodiments in accordance with the present invention relate to a numberof techniques, which may be applied alone or in combination, to reducecharge damage of substrates exposed to electron beam radiation. Inaccordance with one embodiment, charge damage may be reduced byestablishing a robust electrical connection between the exposedsubstrate and ground. In accordance with another embodiment, chargedamage may be reduced by modifying the sequence of steps for activatingand deactivating the electron beam source, thereby reducing chargeaccumulation on the substrate. In accordance with still anotherembodiment, a plasma may intentionally be struck in the chamberfollowing e-beam exposure, thereby removing accumulated charge from thesubstrate. In accordance with still a further embodiment of the presentinvention, voltage on the anode of the e-beam source may be reduced inmagnitude to account for differences in the electron conversionefficiency of different anode materials.

An embodiment of a method in accordance with the present invention ofirradiating a substrate with an electron beam, comprises, disposing asubstrate within a chamber proximate to an anode of an electron beamsource, and placing the substrate into electrical contact with groundthrough a supporting pin. The substrate is then exposed to radiationfrom the electron beam.

An alternative embodiment of a method in accordance with the presentinvention of irradiating a substrate with an electron beam, comprises,disposing a substrate within a chamber proximate to an anode of anelectron beam source, and flowing a processing gas into the chamber fora predetermined time. A bias voltage is applied to the source anodeafter the predetermined time. The substrate is exposed to an electronbeam emitted from a cathode of the electron beam source, by applying ahigh voltage to the source cathode, and delaying regulation of a chamberthrottle valve to adjust a current of the electron beam until after thehigh voltage has been applied to a source cathode for a secondpredetermined time.

An embodiment of a method in accordance with the present invention oftreating a substrate with an electron beam, comprises, disposing asubstrate within a chamber proximate to an anode of an electron beamsource, and applying a bias voltage to the source anode. The substrateis exposed to an electron beam emitted from a cathode of the electronbeam source, and a plasma is introduced into the chamber followingexposure of the substrate to the electron beam.

Another alternative embodiment of a method of irradiating a substratewith an electron beam, comprises, disposing a substrate within a chamberproximate to an aluminum anode of an electron beam source, and applyinga bias voltage to the source anode. A high voltage is applied to analuminum cathode of the electron beam source, such that a voltagedifference between the source anode and the source cathode is betweenabout 1-30 keV.

Another alternative embodiment of an apparatus for treating a substratewith electron beam radiation, comprises, a processing chamber enclosinga substrate support, and an electron beam source comprising an anodeproximate to the substrate support and a cathode distal from thesubstrate support. A ground pin is configured to be in electricalcommunication with an underside of a supported substrate, and inelectrical communication with ground.

These and other objects and features of the present invention and themanner of obtaining them will become apparent to those skilled in theart, and the invention itself will be best understood by reference tothe following detailed description read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified elevational view showing an electron beamexposure apparatus including the presently preferred embodiment of thenew electron source;

FIG. 2 is a fragmentary view similar to FIG. 1, but also showingschematically some of the details of operation;

FIG. 3 is a view similar to FIG. 1, but also showing use of theinvention in shadow mask lithography;

FIG. 4 is a schematic view of another embodiment of the invention, usedas an exposure tool for determining resist sensitivity;

FIG. 5 is a schematic view showing use of the invention in a lift offprocess;

FIG. 6 is a view similar to FIG. 1, and showing the use of feedback tocontrol beam current;

FIG. 7A shows a simplified cross-sectional view of an electron beamexposure chamber in accordance with an embodiment of the presentinvention;

FIG. 7B illustrates a plan view of a support for a substrate within anelectron beam exposure chamber;

FIG. 7C shows a simplified cross-sectional view of an embodiment of agrounded support pin in accordance with the present invention;

FIG. 7D shows a simplified cross-sectional view of an alternativeembodiment of a grounded support pin in accordance with the presentinvention;

FIG. 8 is a simplified flowchart of a turn-on sequence used in analternative embodiment according to the present invention;

FIG. 9 is a simplified flowchart of a turn-off sequence used in anembodiment according to the present invention;

FIG. 10 plots accumulated charge for substrates exposed to e-beamradiation under a number of different conditions;

FIG. 11 plots accumulated charge for substrates exposed to a post-e-beamtreatment plasma discharge; and

FIG. 12 plots accumulated charge for substrates exposed to electron beamradiation under different conditions.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate generally to semiconductorprocessing. Particular embodiments provide a method and structure tocontrol charge buildup in dielectric films. Merely by way of example,the invention has been applied to reducing charge buildup in dielectricfilms after exposure to radiation from a large area electron beam. Themethod and structure can be applied to other applications including, butnot limited to, the control of charge buildup in other materials, suchas semiconductor materials, composite semiconductor/dielectricmaterials, and the like.

U.S. Pat. No. 5,003,178, incorporated herein by reference for allpurposes, describes a design for a large-area uniform electron source.The following additional U.S. patents, describing various applicationsfor electron beam processing, are also incorporated hereby by reference:U.S. Pat. No. 5,468,595, U.S. Pat. No. 6,132,814, U.S. Pat. No.6,204,201, U.S. Pat. No. 6,207,555, U.S. Pat. No. 6,271,146, U.S. Pat.No. 6,319,655, U.S. Pat. No. 6,407,399, U.S. Pat. No. 6,150,070, U.S.Pat. No. 6,218,090, U.S. Pat. No. 6,195,246, U.S. Pat. No. 6,218,090,U.S. Pat. No. 6,426,127, U.S. Pat. No. 6,340,556, U.S. Pat. No.6,319,555, U.S. Pat. No. 6,358,670, and U.S. Pat. No. 6,255,035.

FIG. 1 shows a cross-sectional view of one embodiment of an apparatusfor applying electron beam radiation in accordance with the presentinvention. Cold cathode gas discharge electron source includes a vacuumchamber, indicated by reference numeral 20, a large-area cathode 22, atarget or substrate 30, located in a field-free region 38, and a grid(fine mesh screen) anode 26 placed between the target 30 and cathode 22,at a distance from the cathode that is less than the mean free pathlength of electrons emitted from the cathode. The current can be variedover a wide range by varying the bias voltage applied to grid 26.

The apparatus of the invention further includes a high voltage insulator24, which isolates the grid 26 from the large-area cathode 22, a cathodecover insulator 28 located outside the vacuum, a variable leak valve 32for controlling the pressure inside the vacuum chamber 20, a variablehigh voltage power supply 29 connected to the cathode 22, and a variablelow voltage 0 to −500 volt power supply 31 connected to the grid 26. Insome embodiments, the variable leak valve 32 is referred to as athrottle valve 32.

In operation of the apparatus of the invention, the substrate to beexposed with the electron beam is placed on the target plane 30, and thevacuum chamber 20 is pumped from atmospheric pressure to a pressure inthe range of 1 to 200 milliTorr. The exact pressure is controlled viathe variable rate leak valve 32, which is capable of controllingpressure to +/−1 milliTorr. The high voltage (negative voltage between−500 volts and −30,000 volts or more) at which the exposure is to takeplace is applied to the cathode 22 via the high voltage power supply 29.A variable voltage source 31 (for example: a DC power supply capable ofsourcing or sinking current) is also applied to the grid anode 26. Thevoltage on the grid is utilized to control electron emission from thecathode, as will now be described with reference to FIG. 2.

To initiate electron emission, the gas in the space between the cathode22 and the target 30 must become ionized, producing positive ions 43 andelectrons 45. This transpires as a result of naturally occurring gammarays, or emission can instead be initiated artificially inside thechamber by a high voltage spark gap. Once this initial ionization takesplace, positive ions 43 are attracted to the grid 26 by a slightlynegative voltage (0 to −80 volts) applied to the grid 26. These positiveions 42 pass into the accelerating field region 36 between the cathode22 and the grid anode 26 and are accelerated towards the cathode surface22 as a result of the high voltage applied to the cathode (−500 to−30,000 volts). Upon striking the cathode surface these high energy ionsproduce secondary electrons 44 which are accelerated back toward thegrid 26. Some of these electrons, which are now traveling mostlyperpendicular to the cathode surface, strike the grid (anode) structure26 but many pass through the grid and continue on to the target 30.These high energy electrons ionize the gas molecules in the spacebetween the grid 26 and the target 30.

The fine mesh grid 26 is placed at a distance less than the mean freepath of the electrons emitted by the cathode. Therefore no significantionization takes place in the accelerating field region 36 between thegrid and the cathode. (In a conventional gas discharge device theelectrons emitted would create further positive ions in the acceleratingfield region and all of these ions would be accelerated back to thecathode creating even more electron emission and the discharge couldeasily avalanche into an unstable high voltage breakdown.) However, inthis invention, the ions 42 created outside the grid are controlled(repelled or attracted) by the voltage applied to the grid 26. Thus, theemission (electron beam current) can be continuously controlled (fromvery small currents to very large currents) by varying the voltage onthe grid. Alternatively, the electron emission can be controlled bymeans of the variable leak valve 32, which can raise or lower the numberof molecules in the ionization region between the target and cathode.However, due to the slow response time of adjusting the pressure in thechamber, it is more advantageous to adjust the pressure initially toproduce some nominal emission current and then utilize the bias voltageon the grid 26 to rapidly and precisely control emission current.

The electron emission can be turned off entirely by applying a positivevoltage to the grid 26, such that the positive grid voltage exceeds theenergy of any of the positive ion species created in the space betweenthe grid 26 and target 30. It has been found that the grid can belocated a distance less than 4 mm from the cathode. This distance isless than the mean free path of electrons for the lowest voltage ofinterest (500 volts) and preferred operating vacuum pressure level. Inthe prior art practiced by Induni, he strived for a high vacuum in theaccelerating field region to prevent breakdown.

Fortuitously, the preferred operating vacuum level of this invention isin the region of highest electrical dielectric strength. Therefore, eventhough the grid-to-cathode gap must by necessity be very small to beless than the mean free path determined by the lowest desired operatingaccelerating voltage, the system is operated at a vacuum level where thebreakdown strength of the vacuum exceeds the field created by thehighest operating voltage applied across the selected grid-to-cathodespacing. This low or soft vacuum level (20 to 80 millitorr) is easilyachieved by inexpensive mechanical vacuum pumps and allows the cathode22 and target 30 to be placed in close proximity to each other in thesame vacuum environment.

Further, this mechanism of ion bombardment induced electron emissionmaintains a clean and uniform emitting cathode surface. Although thiscontinual ion bombardment causes erosion of the cathode surface due tosputtering, by utilizing a low sputtering yield cathode material, suchas aluminum, the cathode can be operated continuously for many thousandsof hours without requiring replacement.

The electrons emitted from the cathode 22 are accelerated to the grid 26and are mostly traveling perpendicular to the grid and cathode surface.Some emitted electrons are intercepted by the grid and some arescattered by the grid. If the target 30 is within a few millimeters ofthe grid, the electrons will cast an image of the grid on the target.However, if the target is placed at a large distance 46, such as 10 to20 centimeters from the grid, the electron beam diffuses (due to initialtransverse velocities and scattering) to a fairly uniform currentdensity across the whole emitting area. The irradiation of the targetcan be made even more uniform by sweeping the beam back and forth acrossthe target by means of a time-varying magnetic field produced bydeflection coils 34 surrounding the exposure chamber, as shown in FIG.3.

Reduction in Charge Damage

As described in detail below, the application of electron beam radiationto process substrates may prove useful for a number of applications. Inmany of those applications, the electron beam radiation is applied toinduce a chemical or physical transformation of material on thesubstrate. One unintended consequence of such irradiation, however, maybe the accumulation of charge on the wafer and resulting damage toelectrically active structures present thereon.

Moreover, as the thickness of the dielectric films used as gate oxidesin MOSFET devices has decreased, issues related to gate charge damagehave become more prominent. For example, during the e-beam treatment ofcarbon-doped oxide (CDO) films used as gate oxides, excessive chargebuildup in the dielectric film can lead to gate charge damage. This gatecharge damage can take the form of oxide breakdown, resulting in devicedegradation. Higher leakage currents, as well as a shift in thethreshold voltage, have been observed as a result of gate charge damage.

FIG. 12 plots effective charge for a number of sites on a substratetreated with electron beam radiation. The substrate used in this studyis a Si wafer bearing a high quality thermal oxide 1000 Å thick, whichaccumulated charge after treatment with e-beam radiation under thevarious conditions listed. The total effective charge was measuredutilizing the Quantox XP tool manufactured by KLA Tencor of San Jose,Calif.

As shown in FIG. 12, the effective charge on the wafer increases withthe dose of the applied e-beam radiation. Specifically, the effectivecharge for a wafer receiving a dose of 2000 μC/cm² is considerablylarger than that for the wafer receiving an e-beam dose of 100 μC/cm².For purposes of comparison, FIG. 12 also plots effective charge for areference wafer not receiving any electron beam radiation.

Embodiments in accordance with the present invention relate to a numberof techniques, which may be applied alone or in combination, to reducecharge damage of substrates exposed to electron beam radiation. In oneembodiment, charge damage is reduced by establishing a robust electricalconnection between the exposed substrate and ground. In anotherembodiment, charge damage is reduced by modifying the sequence of stepsfor activating and deactivating the electron beam source to reduce theaccumulation of charge on the substrate. In still another embodiment, aplasma is struck in the chamber containing the e-beam treated substrate,thereby removing charge accumulated by the substrate. In a furtherembodiment of the present invention, voltage of the anode of the e-beamsource is reduced in magnitude to account for differences in electronconversion efficiency exhibited by different anode materials.

A. Substrate Grounding

Referring to FIG. 7A, substrate 30 is in electrical contact with aplurality of pins 70 during processing operations. The substrate isheated, in one embodiment, by quartz halogen lamps 69 and process gasesare supplied through gas inlet 65. The substrate 30 is transferred intoand out of the body of the chamber by a robot blade (not shown) throughan insertion/removal opening (not shown) in the side of the chamber. Themotor raises and lowers the substrate between a processing position anda substrate-loading position (not shown). In the embodiment according tothe present invention illustrated in FIG. 7A, when the substrate is in aprocessing position, at least one of the fixed pins 70 is in electricalcontact with the lower surface of the semiconductor substrate. The fixedpin in electrical contact with the substrate is referred to as a groundpin in a specific embodiment.

The ions 42 created outside the grid 26 are controlled (repelled orattracted) by the voltage applied to the grid 26. Thus, the emission(electron beam current) can be continuously controlled from very smallcurrents to very large currents (i.e. from about 1 mA to about 15 mA) byvarying the voltage on the grid 26. Alternatively, the electron emissioncan be controlled by means of the throttle valve 32 (referring to FIG.1), which can raise or lower the number of molecules in the ionizationregion between the cathode and the substrate. The electron emission canbe turned off entirely by applying a positive voltage to the grid 26,such that the positive grid voltage exceeds the energy of any of thepositive ion species created in the space between the grid 26 and thesubstrate 30.

Conventionally, the ground pin may be in only slidable contact, ratherthan mechanical contact, with the surrounding chamber structure. Such aslip fit of the pin within the chamber may not ensure reliable contactwith ground under vacuum processing conditions, causing the processedsubstrate to electrically float, and allowing it to accumulate charge.

In accordance with one embodiment of the present invention, however, atleast one fixed ground pin 70 c may be placed into reliable electricalconnection with a ground line that communicates with the exterior of thechamber. In one particular embodiment shown in cross-section in FIG. 7Aand in plan view in FIG. 7B, when the wafer is in the processingposition, two of the three pins, 70 a-b, act as a thermocouple (T/C).The third, ground pin 70 c, is in electrical contact with the lowersurface of the semiconductor substrate 30, and is also electricallyconnected with ground line 71 at a point inside the chamber, which, inturn, is routed to exit the chamber and makes electrical connection withthe system ground. Thus, charge that would otherwise build up in thedielectric layers deposited on the wafer surface, is able to dissipateby flowing through the ground pin to the system ground.

FIG. 7C shows a simplified cross-sectional view of an embodiment of animproved ground pin design in accordance with the present invention. Inthe embodiment of FIG. 7C, the bottom portion of the shaft of ground pin70 c bears threads 70 c′ for engagement with the surrounding conductivematerial, thereby ensuring more robust electrical connection between thepin and ground. The embodiment of FIG. 7C is particularly suited forretrofitting of existing chamber designs, wherein a threaded bore can bedrilled to receive the ground pin.

FIG. 7D shows a simplified cross-sectional view of an alternativeembodiment of an improved ground pin design in accordance with thepresent invention. Ground pin 70 c is positioned within bore 70 c″, thathas been expanded through the chamber body to contact the atmosphere.Metal gasket 79 allows for robust electrical contact between the pin andthe grounded body of the chamber.

In accordance with still another alternative embodiment of the presentinvention, the thermocouple (T/C) pins could be utilized to promotegrounding by installing a strap to contact the thermocouple sheath withthe grounded mainframe (M/F) of the tool. In accordance with still otheralternative embodiments of the present invention, a plurality of fixedground pins may be electrically connected to a ground line thatcommunicates with the exterior of the chamber. In these alternativeembodiments, the ground line is electrically connected to the systemground at a location external to the chamber.

B. E-Beam Activation/Deactivation

During processing, a substrate is inserted into the chamber, and thenthe various components of the tool are activated to commenceirradiation, and then deactivated to halt irradiation. For activation,conventionally the lamps are turned on first, and then the Ar gas flowand bias voltage are initiated simultaneously. Next, the high powervoltage supply is turned on, and the beam current established by varyingthe position of the throttle valve.

In accordance with embodiments of the present invention, however, it hasbeen discovered that establishing a flow of the gas simultaneously withapplication of a bias voltage may prove conducive to the formation ofplasma spikes leading to accumulation of charge on the wafer.Accordingly, embodiments in accordance with the present inventionpropose an alternative series of steps to initiate e-beam irradiation.

In one specific embodiment, application of the bias voltage is delayeduntil after the Ar gas is flowed for a brief period. This sequencecreates a stable gas environment for initiation of the plasma.Regulation of the throttle valve to adjust beam current is then delayeduntil after the high voltage has been applied. This combination of stepsserves to suppress plasma spikes

FIG. 8 is a simplified flowchart of a turn-on sequence used in onealternative embodiment according to the present invention. Asillustrated in step 430 of FIG. 8, the transfer robot and wafer liftmechanism are actuated to move the wafer into the processing position.The throttle valve is set at 90° (the full open position) in step 432 toexhaust the maximum volume of gases from the chamber. As will be evidentto those skilled in the art, the order of steps 430 and 432 isinterchangeable as no gas is flowing into the chamber at step 430 asillustrated in FIG. 8.

In step 434, heating elements are turned on to increase the chambertemperature in anticipation of processing operations. In the embodimentillustrated in FIG. 8, quartz halogen heat lamps are used, the set pointis set to a temperature of 397° C., and the final chamber temperaturereaches 400° C. In a particular embodiment, the time required to heatthe chamber to 400° C. is less than 130 seconds. Alternative embodimentsaccording to the present invention utilize increased or decreasedprocessing temperatures, with varying times required to reach thedesired temperature.

After a slight delay, in step 436 a process gas is introduced into thechamber at a predetermined flow rate. In the embodiment illustrated inFIG. 8, the process gas is argon and the flow rate is 100 sccm. In aparticular embodiment, the delay is 5 seconds and the time allotted tostep 436 is 10 seconds. In an embodiment in which the throttle valve isset at 90° and maintained at that position through step 436, the chamberpressure during step 436 is in the range of 12-15 mTorr.

After a delay of time to allow stabilization of voltages and therebysuppress unwanted spiking, in step 438 the bias voltage on the anode isthen set. As discussed in detail below, in certain embodiments the biasvoltage may be set to −125 V, and in other embodiments the bias voltagemay be set at a lower value such as −50 V. In a particular embodiment,the delay is 10 sec. and the time allotted to step 438 is 5 seconds.

In step 440, the cathode voltage (high voltage) is set to −4 KeV and thee-beam is initiated as discussed in reference to FIG. 1.

After a delay to suppress plasma spikes, the throttle valve is partiallyclosed in step 442 to increase the pressure in the chamber and adjustthe e-beam current to a predetermined amount. In a specific embodiment,the throttle valve position is controlled by a system controller toproduce an e-beam current (measured at the cathode) that is equal to 3mA at a high voltage setting of −4 keV and a throttle valve position ofabout 30°. In this embodiment, the chamber pressure, given thesesettings, is in the range of about 20-30 mTorr. In alternativeembodiments, the e-beam current that is produced is equal to 6.0 mA,with a high voltage setting of −6 keV and a throttle valve position inthe range of about 35-40°.

As will be evident to one of skill in the art, the high voltage setting,the throttle valve position, and the e-beam current are related. None ofthese specific e-beam currents are required by the present invention.One of skill in the art will modify these settings to achieve thedesired e-beam processing parameters.

During e-beam treatment of the semiconductor substrate, the exposuredose (e-beam current density multiplied by exposure time) is measured.In a particular embodiment, the exposure dose endpoint is 150 μC/cm². Inan alternative embodiment, the exposure dose endpoint is 1,000 μC/cm².Other embodiments utilize different exposure dose endpoints as necessaryfor their particular processing protocol. In embodiments according tothe present invention, the e-beam processing is continued until apredetermined exposure dose is reached, whereupon the e-beam processingis terminated.

Conventionally, the electron beam irradiation is halted by turning offthe lamps, the Argon gas flow, the bias and high voltages, and the beamcurrent, simultaneously. In accordance with embodiments of the presentinvention, however, it has been discovered that a different sequence ofsteps to halt irradiation may reduce accumulation of charge on thesubstrate.

In accordance with one embodiment of the present invention, the beamcurrent may be turned off first, in order to stop the flow of electronsto the wafer. Next, with the bias voltage remaining on, the high voltageis turned off, thereby allowing a low density plasma to be sustained anddissipate accumulated charge from the wafer. For purposes of this patentapplication, the term “low density plasma” refers to a plasma exhibitinga density of 1×10⁸ ions/cm³ or less. Once the low density plasma hasallowed dissipation of charge, the bias voltage is then turned off,followed by the lamps and the Ar flow.

FIG. 9 is a simplified flowchart of a shut-down sequence used in anembodiment according to the present invention. In step 520, the beamcurrent is turned off.

In one embodiment, the beam current is turned off by modifying thevoltage applied to the grid anode. For example, a positive voltage maybe applied to the grid, such that the positive voltage exceeds theenergy of any of the positive ion species created in the space betweenthe grid 26 and the target workpiece 30 (see FIGS. 1 and 2).

In accordance with an alternative embodiment, the beam current is turnedoff by decreasing the number of molecules in the ionization regionbetween the target and the cathode. For example, opening the throttlevalve to 90° will decrease the number of molecules to a range at whichthe beam current will be turned off.

In a particular embodiment according to the present invention, the timeallotted to step 520 is a predetermined time. For example, in a specificembodiment, the time allotted to step 520 is three seconds. In otherembodiments, the time allotted to step 520 is a greater or lesser time.

In step 522, the high voltage applied to the cathode is turned off. Instep 524, after a slight delay period necessary to allow the low densityplasma resulting from deactivation of the high voltage to dissipatecharge from the wafer, the bias voltage applied to the anode is turnedoff. In embodiments according to the present invention, the timesallotted to steps 522 and 524 are predetermined times. For example, in aspecific embodiment, the time allotted to step 522 is 10 seconds and thetime allotted to step 524 is five seconds. In other embodiments, thetimes allotted to steps 522 and 524 are greater or lesser times.

In step 526, the process gases are turned off. In a specific embodiment,the process gas is argon, the flow of which is terminated in step 526.Additionally, the heating elements are also turned off in this step.Finally, in step 528, the chamber is pumped to the base pressure.

Experimental results have indicated that improved substrate grounding(as described above in conjunction with FIGS. 7A-D), and alteration ofthe activation/deactivation sequence (as described above in conjunctionwith FIGS. 8-9), may serve alone or in combination to beneficiallyreduce accumulation of charge on a wafer. FIG. 10 plots accumulatedcharge (Q_(tot)), under the five sets of conditions indicated. The“Improved Ground” or “New Ground” refers to the threaded ground pinshown in FIG. 7C. The “Improved Sequencing” or “New Sequencing” refersto a process sequence featuring all charged aspects of both theactivated and deactivated sequences of FIGS. 8 and 9. The data shown inFIG. 10 was produced by irradiating a 200 mm wafer at a high power of 4KeV, with a current of 3 mA, a dosage of 1000 μC/cm², at a temperatureof 400° C.

FIG. 10 indicates that use of the improved grounding reduced accumulatedcharge by over 50%. FIG. 10 also indicates that the improved sequencingrecipe reduced charge accumulation by an additional 20% at a dose of1000 μC/cm². At a dose of 100 μC/cm² (not shown in FIG. 10), accumulatedcharge may be reduced even further. In some embodiments, the accumulatedcharge is normalized to provide an accumulated charge density, alsoreferred to as an effective charge, measured in total charge per squarecentimeter.

C. Plasma Discharge

In the modified sequence of e-beam deactivation steps shown in FIG. 9, aresidual low density plasma was exploited to remove accumulated chargeon the wafer. However, embodiments in accordance with the presentinvention are not limited to reducing charge accumulation with such aresidual plasma.

In accordance with alternative embodiments of the present invention,following e-beam exposure, a DC plasma may be purposefully struck in thechamber to provide an increased concentration of ions near thesemiconductor surface, thereby allowing for a reduction in the build-upof charge.

Table 1 shows the value of a number of chamber parameter settings usedduring an experiment conducted by the inventors, in which a substratewas irradiated with e-beam radiation at 400° C., at an energy of 4 KeV,a current of 3 mA, and a dose of 1000 μC/cm². TABLE 1 Parameter (Units)E-Beam Treatment Plasma High Voltage (keV) −4.0 −0.5 Bias Voltage (V)−125 −150 Argon Flow (sccm) 100 100 Throttle Valve Position (°) 30 15Chamber Pressure (mTorr) 35 60 Process Time (s) — 30

The second column shows the chamber settings used during an e-beamtreatment process and the third column shows the chamber settings usedduring a subsequent low density plasma treatment process. Referring toTable 1, the last row of the second column is blank because the e-beamtreatment process was terminated after a predetermined exposure dose (inthis experiment 1,000 μC/cm²) was reached, not after a specific periodof time had elapsed.

Results from the various portions of the experiment are shown in FIG.11, which plots total charge buildup (Q_(tot)) per cm² for threecases: 1) after e-beam treatment (without low density plasma treatment);2) after e-beam treatment followed by low density plasma treatment; and3) a reference/control of no e-beam or low density plasma treatment.

FIG. 11 indicates that the charge buildup in the dielectric was reducedfrom 297.5×10¹⁰ eV⁻¹cm⁻² to 31.35×10¹⁰ eV⁻¹cm⁻² following the plasmatreatment. This level of accumulated charge after plasma treatmentcompares favorably with the reference charge of 26.3×10¹⁰ eV⁻¹cm⁻² thatwas measured in the absence of any application of e-beam irradiation. Inone embodiment, accumulated charge density is measured in units of totalcharge per square centimeter. In another embodiment, interface trapdensity is measured in units of eV⁻¹cm⁻². Both measurements techniquescan be employed in measuring reductions in charge density provided byembodiments in accordance with the present invention.

According to the embodiment depicted in Table 1, the high voltageapplied to the cathode during the post-irradiation plasma treatment isset at −0.5 keV. In alternative embodiments, the high voltage may be setto a lower value, including zero. Similarly, the bias voltage applied tothe anode during the low density plasma treatment is set at −150 V. Inalternative embodiments, the bias voltage may range to about −500 V,encompassing such possible bias voltage settings such as −50 V, −100 V,and −125 V.

As will be evident to one of skill in the art, the flow rate of theprocess gas, (here argon), the throttle valve position, and the chamberpressure are related. Accordingly, an increase in the gas flow rate or adecrease in the throttle valve position will increase the chamberpressure. An increase in chamber pressure will result in an increase inthe number of ions available to dissipate charge buildup in thedielectric materials present on semiconductor surface. Thus while theprocess time in this embodiment is 30 seconds, this specific value isnot required by the present invention, and the plasma exposure time willoperate as a function of the number of available ions, among otherfactors.

D. Reduced Anode Voltage

As described in detail above, a bias voltage applied to the anode servesto control electron emission from the cathode. Conventionally, a biasanode voltage of −125 V has been applied to an aluminum anode. However,this magnitude of anode voltage has been observed to result in arcingevents between the anode and the substrate, thereby contributing tounwanted accumulation of charge.

Moreover, the −125 V value for the anode voltage was based upon priorapplications involving a large area e-beam source operated at roomtemperature and having a graphite anode. Such graphite anodes, however,are not suitable for higher temperature applications in accordance withthe present invention, and have been replaced with anodes comprisingaluminum. Another possible example of a material comprising the anode istitanium.

The aluminum material of the anode utilized by a large area electronbeam source in accordance with the present invention exhibits adifferent secondary yield coefficient than the graphite material of theconventional anodes. Specifically, when ions bombard the anode (andcathode), they generate secondary electrons. The number of secondaryelectrons that are generated depend on the secondary electron yieldcoefficient. If the coefficient is 1.0, then the generation of secondaryelectrons is 100%. Graphite exhibits a secondary yield coefficient ofabout 0.086, while aluminum exhibits a secondary yield coefficient ofabout 0.125. Given that graphite has a lower secondary electron yieldcoefficient than Al, when graphite is used as the anode, a highervoltage needs to be used to generate the same number of secondaryelectrons.

The improved secondary yield coefficient of aluminum relative tographite allows a reduced bias voltage to be applied to the anode of thelarge area electron. For example, for a high (cathode) voltage of 4 keVor greater, the bias (anode) voltage may range from about −50 V to about−500 V. At lower cathode voltages, the upper limit of the range ofallowable anode voltages would be reduced.

Possible Applications

Embodiments of apparatuses and methods for electron beam exposure inaccordance with the present invention may be employed in a variety ofapplications. One such application is in the curing of deposited low Kfilms to form ultra low K nanoporous films. U.S. Pat. No. 6,541,367,incorporated by reference herein for all purposes, describes one methodof forming such a nanoporous film. Another such method is described inU.S. Pat. No. 6,596,627, also incorporated by reference herein for allpurposes. E-beam processing is more fully described in U.S. patentapplication Ser. No. 10/302,375, entitled, “Method For Curing LowDielectric Constant Film By Electron Beam”, filed on Nov. 22, 2002,incorporated by reference herein for all purposes.

Other embodiments of the disclosed invention can be used for shadow masklithography. An aperture plate or mask 48 is placed between the grid 26and in contact or close proximity with the target 30, as shown in FIG.3. Since the electrons moving toward the target 30 are nearly collimatedby the accelerating field, as indicated at 50, and have relatively smalltransverse velocities, a shadow mask, such as the plate 48, placed inclose proximity to the target will be accurately replicated by theelectron beam 52 that is allowed to pass through the mask or apertureplate. In this way patterned lithography can be performed using theprinciple of the invention.

In yet another embodiment of the invention, used as a resist sensitivitytool as shown in FIG. 4, a shaped aperture 54 is placed between the gridand target. This aperture can form a small shaped electron beam having auniform current density. The target material is then scanned or steppedunder the beam to generate multiple patterns in an electron sensitiveresist coated on the substrate or target. The electron beam passesthrough the shaped aperture 54 and impinges on a target substrate,indicated at 56 in FIG. 4, which is mounted on a movable slide 58, whichis in turn, mounted on base 53. After exposing a square feature 59 ofthe substrate 56, the slide 58 is moved by means of a leadscrew drive 60and crank 62. The crank motion is coupled into the vacuum system by asuitable rotary mechanical vacuum feedthrough (not shown). The substrate56 is moved over enough to expose a new area of resist 61. Multipleexposures are made at different selected exposure doses and acceleratingvoltages. This technique has proved to be a very useful tool inevaluating the sensitivity of resist. By exposing a series of squarepatterns across a substrate, with each square having a slightlydifferent level of exposure, resist sensitivity curves can be quicklyestablished. Prior to the present invention, this could only be donewith a very expensive electron beam lithography system. In thisapplication, it is important to provide a very precise and uniformexposure in each feature exposed. It has been found by deflecting thebeam above the aperture, utilizing magnetic deflection coils 34, thatmore uniform exposures can be achieved. The deflection coils scandifferent portions of the cathode emitting area over the pattern formingaperture, thereby averaging any nonuniformities in cathode emission.Since the aperture size is known, the exposure dose is determined with asimple electronic integrator 66, which measures the total integratedcurrent reaching the substrate. The substrate is electrically connectedto the integrator, which consists of a capacitor 68, operationalamplifier 67, and voltmeter 72. The current collected by the substratetends to charge the capacitor 68 through a feedback loop. The invertinginput 74 of the operational amplifier 70 is a virtual ground referencedto the non-inverting input 78. The voltage at the amplifier output 76 isrelated to the dose by the expression D=EC/A, where D is the exposuredose in Coulombs per square centimeter, E is the voltage at the output76, C is the capacitance in Farads of the capacitor 68, and A is thearea in square centimeters of the aperture 54. The advantage of thismethod of dose control is that it measures actual dose in real time. Inconventional electron beam lithography systems, the exposure dose isindirectly determined by the time of exposure and independentmeasurement of beam current before or after the actual exposure.

Another application of this invention is to resist curing. Insemiconductor fabrication after pattern lithography has been performed,a resist layer must be hardened or cured prior to etching. Conventionalpractice utilizes baking of the resist to a high temperature. However,at these elevated temperatures the resist melts slightly and the patternareas become distorted. Electron beam exposure of the resist provides anonthermal means of crosslinking and hardening the resist. The substratestays at room temperature yet the resulting exposed resist is fullycrosslinked without pattern flow. With this invention, resist curing canbe faster than ultraviolet curing or baking and results in a tougherresist film. In addition the electron beam can cure very thick resists,up to 20 micrometers at 30 KeV, which cannot be cured using ultravioletcuring systems. The ultraviolet radiation is absorbed in surface layersof the resist. Prior to this invention, electron beam curing of resisthas not been widespread, due to the cost and time required byconventional electron beam lithography systems. With this new approach,using an inexpensive electron source as described, electron beam curingbecomes a favorable alternative to baking or ultraviolet curing.

Another application of the disclosed invention is to provide an easilymodulated electron beam source for lithography. In most electron beamlithography systems the electron beam is at high energy and is noteasily turned on (unblanked) and off (blanked). To accomplish blankingin prior art systems the beam is deflected off an aperture in theelectron optical column. However, there are drawbacks to this approach:the beam at the target plane moves while blanking occurs causingunwanted anomalies in the patterns being written. In addition the beam'scontinuous bombardment on the blanking aperture causes contamination andcharging of the aperture deflecting the beam and causing errors inpositioning of patterns being written. In practicing the presentinvention, it has been found that, at lower vacuum levels than practicedin prior art systems, electron emission has been achieved by biasing theanode aperture or grid 26, and further that a high energy beam >30 KeVcan be turned on and off with just a few (1 to 5) volts variation on thegrid. This small voltage on the exit aperture or grid anode hasvirtually no effect on the beam's landing position. This permits thiselectron source to be utilized in high resolution electron beamlithography and pattern generation as well as other applicationsrequiring a modulated electron beam such as electron beam testing andinspection of integrated circuit devices.

Another very useful application of the disclosed invention is as an aidin lift off processing as used in semiconductor fabrication. Lift offtechniques in depositing patterned metal films have become quitewidespread in semiconductor processing. As shown in FIG. 5, a substrate82 to be patterned is coated with a photoresist 84, and exposed anddeveloped using conventional photolithography. The metal to be depositedon the substrate is evaporated or sputtered on top of the resist film84, as indicated at 86, and directly to the substrate 82 in developedwindows in the resist, as indicated at 88.

At this point in the process, all that remains is to dissolve the resistremaining, i.e. in areas under the metal at 86, which will leave themetal film at areas 88, in the selected patterned areas only. However,this is the most difficult step in lift off processing, because themetal film covers the resist and keeps the solvent from dissolving theunderlying resist. One proposed solution to this problem is to employ ahigh-power laser to disrupt the metal film over the resist. However, abetter technique is to employ the new electron source of the presentinvention. By utilizing a broad area electron beam 80, it is possible torender the underlying resist 84 more soluble by exposing it with theelectron beam at an appropriately high energy, such as 30 KeV. Inaddition to making the underlying resist more soluble, with largeexposure doses (200 μC/cm²) the metal film 86 tends to blister, allowinga solvent to reach the underlying resist 84 in the subsequentdissolution process step. Although this technique may have been possibleusing conventional electron beam pattern generation systems, it was notpractical because of the large exposure dose required over the entiresubstrate.

A further embodiment of this invention is shown in FIG. 6. In someapplications it may be desirable to provide a constant beam current atdifferent electron beam energies. For example it may be desirable toexpose or cure the upper layer of resist on a resist coated substrate,but not the bottom layer. This can be done by utilizing an electron beamenergy low enough such that most of the electrons are absorbed in theupper layer of the resist. Subsequent to curing the top layer, it may bedesirable to cure the full thickness of the resist layer. This can bedone by raising the accelerating voltage of electron beam to penetratecompletely through the resist layer to the substrate. It would bedesirable in performing these exposures to be able to alter theaccelerating voltage without causing a change in the emission current.However, if the accelerating voltage is increased it tends to cause moreionization and therefore an increase in beam current. Similarly if theaccelerating voltage is lowered, ionization lessens and the beam currentis decreased. A means of maintaining a constant beam current independentof changes in accelerating voltage is shown in FIG. 6. The beam currentis sampled via a sense resistor 90, which is placed between the targetand the integrator 66. (Alternatively, the beam current could be sampledat the grid as a portion of the beam is intercepted there.) Two unitygain voltage followers 92 buffer the signal obtained across the senseresistor 90 and feed it to an amplifier 96 with adjustable gain throughvariable resistor 94. The output of this amplifier controls the voltageon the grid anode 26, such that an increase in beam current will cause adecrease in bias voltage on the grid and decrease in emission currentfrom the cathode 26. The gain of the amplifier 96 is adjusted, by meansof the variable resistor 94, so that any change in current caused by achange in the accelerating voltage is counteracted by a change in biasvoltage to maintain the beam current reaching the target constant.Alternatively, the output of the amplifier 96 can be connected to avoltage controlled variable rate leak valve 98, to counteract changes inemission current by raising or lowering the pressure in the field freeregion 38 where ionization occurs. Further, a wider range of beamcurrent control could be provided by utilizing feedback signals to boththe variable leak valve 98 and the grid 26.

It will be appreciated from the foregoing that the present inventionrepresents a vast improvement over other electron sources. Inparticular, the electron source of the invention provides a uniform,large-area beam of electrons at an easily controlled current level.Moreover, beam uniformity and beam current control are effective over awide range of beam accelerating voltages, and under relatively poorvacuum conditions.

It will also be appreciated that, although various embodiments of theinvention have been described in detail for purposes of illustration,various modifications may be made without departing from the spirit andscope of the invention. Accordingly, the invention is not to be limitedexcept as by the appended claims.

Merely by way of example, the invention has been applied to reducingcharge buildup in dielectric films that are exposed to radiation fromlarge area electron beam sources. The method and structure can beapplied to other applications including, but not limited to, the controlof charge buildup in other materials, such as semiconductor materials,composite semiconductor/dielectric materials, and the like.

While the above is a complete description of specific embodiments of thepresent invention, various modifications, variations, and alternativesmay be employed. These equivalents and alternatives are included withinthe scope of the present invention. Therefore, the scope of thisinvention is not limited to the embodiments described, but is defined bythe following claims and their full scope of equivalents.

1. A method of irradiating a substrate with an electron beam, the method comprising: disposing a substrate within a chamber proximate to a source anode of an electron beam source; placing the substrate into electrical contact with ground through a supporting pin; and exposing the substrate to radiation from the electron beam.
 2. The method of claim 1 wherein the supporting pin is provided in threaded engagement with a grounded chamber body.
 3. The method of claim 1 wherein the supporting pin is provided in physical contact with a metal gasket of the chamber.
 4. The method of claim 1 wherein: exposing the surface of the substrate to electron beam radiation comprises delaying application of a bias voltage to the source anode until a process gas has flowed into the chamber for a predetermined time; and the method further comprises delaying regulation of a chamber throttle valve to adjust a current of the electron beam until a high voltage has been applied to a source cathode for a second predetermined time.
 5. The method of claim 1 further comprising introducing a plasma into the chamber following exposure of the substrate to the electron beam.
 6. The method of claim 5 wherein the plasma is intentionally struck in the chamber.
 7. The method of claim 5 wherein the plasma is residual from a delay between deactivation of the high voltage to the source cathode, and deactivation of the bias voltage to the source anode, while processing gas is flowed into the chamber.
 8. The method of claim 1 wherein exposing the substrate to radiation from the electron beam comprises applying a voltage difference of about 3.05 KeV or less between the source anode comprising aluminum and a source cathode comprising aluminum.
 9. The method of claim 1 wherein exposing the substrate to radiation from the electron beam comprises applying a voltage difference of about 2.95 KeV or less between the source anode comprising aluminum and a source cathode comprising aluminum.
 10. The method of claim 8 wherein a high voltage of about +3 KeV is applied to the source cathode, and a bias voltage of about −50 V is applied to the source anode.
 11. The method of claim 8 wherein a high voltage of about −3 KeV is applied to the source cathode, and a bias voltage of about −50 V is applied to the source anode.
 12. A method of irradiating a substrate with an electron beam, the method comprising: disposing a substrate within a chamber proximate to an anode of an electron beam source; flowing a processing gas into the chamber for a predetermined time; applying a bias voltage to the anode after the predetermined time; and exposing the substrate to an electron beam emitted from a cathode of the electron beam source, by applying a high voltage to the cathode, and delaying regulation of a chamber throttle valve to adjust a current of the electron beam until after the high voltage has been applied to the cathode for a second predetermined time.
 13. The method of claim 12 further comprising introducing a plasma into the chamber following exposure of the substrate to the electron beam.
 14. The method of claim 13 wherein the plasma is intentionally struck in the chamber.
 15. The method of claim 13 wherein the plasma is residual from a delay between deactivation of the high voltage to the cathode, and deactivation of the bias voltage to the anode, while the processing gas is continued to be flowed into the chamber.
 16. The method of claim 12 wherein exposing the substrate to radiation from the electron beam comprises applying a voltage difference of about 3.05 KeV or less between the anode comprising aluminum and the cathode comprising aluminum.
 17. The method of claim 12 wherein exposing the substrate to radiation from the electron beam comprises applying a voltage difference of about 2.95 KeV or less between the anode comprising aluminum and the cathode comprising aluminum.
 18. The method of claim 16 wherein a high voltage of about +3 KeV is applied to the cathode, and a bias voltage of about −50 V is applied to the anode.
 19. The method of claim 16 wherein the high voltage of about −3 KeV is applied to the cathode, and the bias voltage of about −50 V is applied to the anode.
 20. A method of treating a substrate with an electron beam, the method comprising: disposing a substrate within a chamber proximate to an anode of an electron beam source; applying a bias voltage to the anode of the electron beam source; exposing the substrate to an electron beam emitted from a cathode of the electron beam source; and introducing a plasma into the chamber following exposure of the substrate to the electron beam.
 21. The method of claim 20 wherein the plasma is intentionally struck in the chamber.
 22. The method of claim 20 wherein the plasma is a low density plasma residual from a delay between deactivation of a high voltage to the cathode of the electron beam source, and deactivation of the bias voltage to the anode of the electron beam source, while a processing gas is continued to be flowed into the chamber.
 23. A method of irradiating a substrate with an electron beam, the method comprising: disposing a substrate within a chamber proximate to an aluminum anode of an electron beam source; applying a bias voltage to the aluminum anode; and applying a high voltage to an aluminum cathode of the electron beam source, such that a voltage difference between the aluminum anode and the aluminum cathode is between about 1-30 keV.
 24. The method of claim 23 wherein the bias voltage of about −50 eV is applied to the aluminum anode, and the high voltage of about +3 KeV is applied to the aluminum cathode.
 25. The method of claim 23 wherein the bias voltage of about −50 V is applied to the aluminum anode, and the high voltage of about −3 KeV is applied to the aluminum cathode.
 26. The method of claim 23 wherein the bias voltage is less than 125 V.
 27. An apparatus for treating a substrate with electron beam radiation, the apparatus comprising: a processing chamber enclosing a substrate support; an electron beam source comprising an anode proximate to the substrate support and a cathode distal from the substrate support; and a ground pin configured to be in electrical communication with an underside of a supported substrate, and in electrical communication with ground.
 28. The apparatus of claim 27 wherein the ground pin is in physical contact with a chamber ground.
 29. The apparatus of claim 27 wherein the substrate support is grounded, and the ground pin is in threaded engagement with the substrate support.
 30. The apparatus of claim 27 wherein the ground pin is in electrical communication with a grounded metal gasket located on an exterior of the chamber. 